The present invention relates to engine exhaust systems and particularly to exhaust catalyst systems. More particularly the invention relates to catalyst units.
Spark ignition engines often use catalytic converters and oxygen sensors to help control engine emissions. A gas pedal is typically connected to a throttle that meters air into engine. That is, stepping on the pedal directly opens the throttle to allow more air into the engine. Oxygen sensors are often used to measure the oxygen level of the engine exhaust, and provide feed back to a fuel injector control to maintain the desired air/fuel ratio (AFR), typically close to a stoichiometric air-fuel ratio to achieve stoichiometric combustion. Stoichiometric combustion can allow three-way catalysts to simultaneously remove hydrocarbons, carbon monoxide, and oxides of nitrogen (NOx) in attempt to meet emission requirements for the spark ignition engines.
Compression ignition engines (e.g., diesel engines) have been steadily growing in popularity. Once reserved for the commercial vehicle markets, diesel engines are now making real headway into the car and light truck markets. Partly because of this, federal regulations were passed requiring decreased emissions in diesel engines.
Many diesel engines now employ turbochargers for increased efficiency. In such systems, and unlike most spark ignition engines, the pedal is not directly connected to a throttle that meters air into engine. Instead, a pedal position is used to control the fuel rate provided to the engine by adjusting a fuel “rack”, which allows more or less fuel per fuel pump shot. The air to the engine is typically controlled by the turbocharger, often a variable nozzle turbocharger (VNT) or waste-gate turbocharger.
Traditional diesel engines can suffer from a mismatch between the air and fuel that is provided to the engine, particularly since there is often a time delay between when the operator moves the pedal, i.e., injecting more fuel, and when the turbocharger spins-up to provide the additional air required to produced the desired air-fuel ratio. To shorten this “turbo-lag”, a throttle position sensor (fuel rate sensor) is often added and fed back to the turbocharger controller to increase the natural turbo acceleration, and consequently the air flow to the engine.
The pedal position is often used as an input to a static map, which is used in the fuel injector control loop. Stepping on the pedal increases the fuel flow in a manner dictated by the static map. In some cases, the diesel engine contains an air-fuel ratio (AFR) estimator, which is based on input parameters such as fuel injector flow and intake manifold air flow, to estimate when the AFR is low enough to expect smoke to appear in the exhaust, at which point the fuel flow is reduced. The airflow is often managed by the turbocharger, which provides an intake manifold pressure and an intake manifold flow rate for each driving condition.
In diesel engines, there are typically no sensors in the exhaust stream analogous to that found in spark ignition engines. Thus, control over the combustion is often performed in an “open-loop” manner, which often relies on engine maps to generate set points for the intake manifold parameters that are favorable for acceptable exhaust emissions. As such, engine air-side control is often an important part of overall engine performance and in meeting exhaust emission requirements. In many cases, control of the turbocharger and EGR systems are the primary components in controlling the emission levels of a diesel engine.
Most diesel engines do not have emissions component sensors. One reason for the lack of emissions component sensors in diesel engines is that combustion is about twice as lean as spark ignition engines. As such, the oxygen level in the exhaust is often at a level where standard emission sensors do not provide useful information. At the same time, diesel engines may burn too lean for conventional three-way catalysts.
After-treatment is often needed to help clean up diesel engine exhaust. After-treatment often includes a “flow through oxidation” catalyst. Typically, such systems do not have any controls. Hydrocarbons, carbon monoxide and most significantly those hydrocarbons that are adsorbed on particulates can sometimes be cleaned up when the conditions are right. Other after-treatment systems include particulate filters. However, these filters must often be periodically cleaned, often by injecting a slug of catalytic material with the fuel. The control of this type of after-treatment may be based on a pressure sensor or on distance traveled, often in an open loop manner.
Practical NOx reduction methods presently pose a technology challenge and particulate traps often require regeneration. As a consequence, air flow, species concentrations, and temperature should be managed in some way in order to minimize diesel emission levels.
Development of exhaust catalyst systems has been useful for meeting engine emissions requirements around the world. There has been a need for emission reduction efficiency and improved fuel economy in such developed catalyst systems.